U.S. Pat. No. 4,701,052 (Schoen) discloses a dew point hygrometer that uses linearly-polarized light to detect condensate on a metal surface. Schoen declares the presence of condensation when a light detector receives no light from an analyzer.
U.S. Pat. No. 4,725,145 (Azzam) discloses a polarimeter from which one parameter of polarization of a light beam can be determined. The Azzam invention is a polarization sensitive photodetector that produces an output proportional to the radiation absorbed by the photodetector.
U.S. Pat. No. 4,668,860 (Anthon) discloses a scattermeter for evaluating the surface quality of an optical element, wherein surface scatter is distinguished from bulk scatter by differing polarization characteristics. Neither Azzam nor Anthon incorporate the intrinsic birefringent properties of the hexagonal crystalline structure of the ice crystal.
Limitations of the prior art reveal the need for an accurate and reliable ice detection system. The present invention uses the intrinsic birefringent properties of the hexagonal ice crystal to detect ice by analyzing the elliptically-polarized reflection of linearly-polarized radiation on ice using a plurality of elemental sensors--the determination of ice being a function of the variance among the output signals of the sensors. The ice detection system of the present invention is advantageous over the prior art because it provides more information through a qualitative measurement, and is more accurate because it is based on multiple inputs.